Various types of evaporators are used to concentrate liquids or to recover solvents, e.g. from solutions, suspensions, emulsions or the like. In the evaporator, the concentration of the several substances in the liquid is increased and part of the evaporated liquid flows through a separator, in which condensed constituents are removed, to another evaporator stage, whereas the condensate is returned through downcomers. The concentrate which is still pumpable may be the end product, or this end product may be discarded and only the vapors recovered, e.g. as fresh water produced by a distillation of sea water.
In most evaporators, a heater, e.g. a steam-heated tube-type heat exchanger, is used to heat the liquid to be treated to the boiling temperature so that the liquid evaporates (Grundlagen der chem. Technik, Vol. 4, Heat Exchangers, by Roman Gregory, 1959, Verlag H. R. Sauerlander, Aarau).
In so-called natural-circulation evaporators, a so-called "apparent liquid level" results during operation. This apparent liquid level is detected by a liquid-level indicator, which is connected to the liquid space below the heating tubes and to the vapor space above the heating tubes. For instance, when the liquid-level indicator of a circulating evaporator having heating tubes 2 m long indicates that the tubes are "half filled", this means that the pressure at the tube inlet exceeds the pressure at the outlet by a head of 1 m of the liquid. The fluid rising from the tubes can be delivered by the downcomers to the chamber below the heating tubes where the pressure is 1 m of liquid column higher than the boiling pressure.
If an evaporator for evaporating water from an aqueous solution is operated at an evaporating temperature of 50.degree. C., a pressure of 0.125 kg/cm.sup.2 (absolute pressure of 0.125 atm) will prevail in the evaporator space above the heating tubes. In this example the pressure at the inlet of the tubes will be 0.125 kg + 0.1 kg = 0.225 kg per square centimeter (absolute pressure) or 0.125 atm (abs) + 0.1 atm (abs) = 0.225 atm (abs) if it is assumed that the specific gravity of the material to be concentrated equals unity. Without an increase of the boiling point, an aqueous solution under 0.225 kg/sq.cm (absolute pressure) or 0.225 atm (abs) boils at 61.degree. C. so that the material to be concentrated cannot boil at the inlet of the tubes. Because the material to be concentrated rises in the parallel heating tubes at a velocity of only about 1 cm/sec., it is preheated along a fairly long path before the evaporation in the heating tubes can begin.
Owing to the low velocity of flow, there is only a small heat transfer in the preheating zone and depositions of solids on the heating surface tend to take place preferentially in this zone and may further reduce the heat transfer.
Flowthrough evaporators for cascade operation comprise evaporator segments or evaporators which are connected in series for the flow of liquid. When the material to be concentrated has risen in heating tubes, it flows through downcomers to other heating tubes in which the material rises again as it is evaporated. The weight of the liquid in the heating tubes, the acceleration of the material to be concentrated and of the vapor, and the pipe friction result in a pressure difference between the heating-tube inlet disposed on a lower level and the heating-tube outlet disposed on an upper level. In heating tubes 4 m long, for instance, the pressure difference between the upper and lower ends may amount to 2 m of liquid. With an evaporating temperature of 50.degree. C., an aqueous solution to be concentrated begins to boil at the tube inlet only at 71.degree. C. The liquid entering the heating tubes at about 1 cm/sec must be preheated if the temperature rise and pressure drop are to permit the evaporation to begin during the upward flow.